0258-Add-Digits.py

'''
Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

Example:

Input: 38
Output: 2 
Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2. 
             Since 2 has only one digit, return it.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?
'''
class Solution:
    def addDigits(self, num: int) -> int:
        if num < 10:
            return num
        return num % 9 if num % 9 != 0 else 9